1. Field of the Invention
The present invention, relates to a gradient amplifier for a magnetic resonance (MR) tomography apparatus of the type having a control unit connected upstream to a modulator stage before the output stage for current control, the control unit including a comparison stage for the reference (or mominal) value and the actual value of the respective gradient coil current, and having a P control branch that is connected downstream and an I control branch that is situated at the output of the control branch, the outputs of the control branches being conducted to an output sum stage via calibrating resistors.
2. Description of the Prior Art
A gradient amplifier of this type is known from the German OS 197 06 756. Aside from the relatively low ohmic resistance losses, the gradient amplifier operates on a purely inductive load. The current regulator of the amplifier already regulates the system deviation, arising during a pulse edge to zero, although a short-term overshoot of 5 to 10% of the amplitude of the pulse peak occurs depending on the switching frequency, the output stage voltage, the edge steepness and the control setting. This results in an undesirable deformation of the pulse.
In order to avoid this, an optimally high switching frequency of the output stage is desirable and a very good controller setting is necessary. Furthermore, eddy current compensation is used to minimize the undesirable deformation of the pulse, although eddy current compensation is designed not for this purpose. Despite a considerable outlay, all these measures still cannot eliminate the undesirable pulse deformations entirely.
An object of the present invention is to provide a gradient amplifier of the above type wherein undesirable pulse deformations are prevented to a large extent with only low circuitry outlay.
This object is inventively achieved in a gradient amplifier wherein a voltage is produced that relieves the controller and that is proportional to the pulse edge of the gradient coil current, this voltage being added to the controller output.
The control circuit is thus inventively combined with an additional control, whereby the control circuit is largely relieved by this additional control and then has to provide only a fine control and for the application of the resistive portion.
For this purpose in an embodiment of the invention the output of an additional differential control stage, which stage is controlled directly by the current reference value, is applied at the input of the output sum stage via a calibrating resistor.
The differential control stage preferably contains an operating amplifier, to which a resistor is connected in parallel, and which is controlled by the current reference value signal at the input of the comparison stage via a capacitor.
Analogously to the construction of the control branches, an adjustable amplifier can be connected to the operating amplifier downstream.